CD59 is a key regulator for restricting the formation of the membrane attack complex (MAC) of complement by binding to complement components C8 and C9 and preventing C9 incorporation and polymerization in the cell membrane. Complement is a key mediator for antibody-based cancer therapy such as complement-dependent cytotoxicity (CDC) and antibody-dependent-cell mediated cytotoxicity (ADCC). CD59 is universally expressed in all cells and highly expressed in many kinds of cancer cells including B-cell lymphoma. CD59 appears to be the membrane complement regulator that is most effective at protecting tumor cells from antibody-mediated CDC. Extensive studies indicate that upregulation of CD59 is responsible for lymphoma resistance to rituximab treatment. Therefore, it is imperative for us to develop a molecule capable of abrogating CD59 function in cancer cells and facilitating antibody-mediated cancer therapy. However, targeted toxicity effect from miniantibody specific against hCD59 and less efficacy of C8 or C9 peptides limit them for therapeutic purposes. Intermedilysin (ILY), a cytolytic pore-forming toxin secreted by Streptococcus intermedius, lyses only human cells due to its receptor specificity for human CD59 (hCD59). Recently, we used hCD59 transgenic mice to confirm the finding that ILY binds only to hCD59 in vivo. Domain 4 of ILY binds to AA42-58 in hCD59, which also participate in the binding to C8 and C9. Thus, we hypothesize that the truncated ILY presenting only domain 4 will specifically abrogate hCD59 function and facilitate antibody- mediated and complement-dependant cancer cytolysis. Consistently, our preliminary results have demonstrated that the recombinant protein (114AA) derived from the ILY domain 4 (rILYd4) specifically blocks hCD59 function ex vivo. Furthermore, we demonstrated that application of rILYd4 (IC50 = 33 nM in vitro and effective dose = 2.5 ug/g body weight in vivo ) to B lymphoma cell lines (RAMOS), which are resistant to Rituximab-mediated CDC (so called RRR cells), sensitized them to Rituximab-mediated CDC in lymphoma without off target toxicity effects. We propose to use rituximab as a therapeutic antibody in lymphoma xenografted athymic nude mice that express transgenic hCD59 to test this hypothesis. Specifically, we will determine and the efficacy and specificity of rILY4 (specific aim 1) and characterize rILYd4 immunogenicity (Aim 2). Success in this application will yield a newly-identified CD59 inhibitor, domain 4 of ILY as an adjunct to not only rituximab-based lymphoma therapy, but also other antibody-based cancer therapy. PUBLIC HEALTH RELEVANCE: In this application, we will assess the in vivo efficacy of the nature product rILYd4 for antibody (rituximab)-mediated cancer (B-lymphoma) therapy. Success of this work will foster us to develop novel anti-cancer reagents, an anti-human CD59 inhibitor rILYd4, which can significantly facilitate antibody- mediated cancer therapy, not only in lymphoma, but also in other cancers.